Insights into the Nature and Thermal Phase Behavior of Mechanochemically Prepared Multicomponent Solids of 2-Amino-5-nitropyridine with para -Substituted Benzoic Acids

Abstract

The thermal phase behavior of binary systems consisting of the supramolecular building block 2-amino-5-nitropyridine (ANP) with five systematically varied series of para-substituted aromatic acids (4XBA), viz, benzoic acid (BA), 4-fluorobenzoic acid (4FBA), 4-chlorobenzoic acid (4CBA), 4-bromobenzoic acid (4BBA), and 4-iodobenzoic acid (4IBA), was investigated separately. While single-crystal structures of 1:1 ANP-4FBA, 1:1 ANP-4CBA, and 1:1 ANP-4BBA in this series are known, their bulk thermodynamic stability and mechanochemical phase behavior have remained unmapped. The bulk binary mixtures of various compositions were prepared by an environmentally friendly mechanochemical neat grinding method. The thermal properties of the ground materials were thoroughly mapped using differential scanning calorimetry (DSC). The solid−liquid thermal binary phase diagrams were systematically constructed using DSC data and validated using Tammann plots and Predictor of Eutectic Mixture (PoEM) analysis. The phase diagram study of all the five systems revealed the distinct thermal phase behavior across the series. The ANP-4FBA system forms a stable congruently melting 1:1 cocrystal characterized by a “W-shaped” phase diagram with two eutectic valleys, while the ANP-BA, ANP-4CBA, and ANP-4BBA systems form an incongruent 1:1 cocrystal characterized by an asymmetrical liquidus curve terminating at a horizontal peritectic isothermal line in their phase diagrams. Notably, the ANP-4IBA system fails to form a cocrystal entirely, resulting in a simple eutectic mixture with an ANP composition (in mole fraction) of 0.72. The experimental analyses using powdered X-ray diffraction (PXRD) and Fourier Transform Infrared Spectroscopy (FTIR) confirmed the nature of solids obtained. For ANP-BA, ANP-4CBA, and ANP-4BBA, thermogravimetry analysis (TGA) provided complementary support for the DSC-derived phase diagram predictions of incongruent melting.